Kevin Nguyen knows exactly what kind of character would be the hero in his first video game.

The 17-year-old, who dreams of becoming a video game designer, imagines not a muscular warrior but a courageous young man who, through his adventures, seeks to sort out his uncertain past.

That describes Kevin, who has fought a daily battle with illness and infection for most of his life. His immune system is being destroyed by Hyper IgM Syndrome, a rare genetic disease that killed his two brothers.

The disease has kept Kevin out of school. He has been learning with a tutor three times a week and will accept a certificate today from McKinley High School for completing a special educational track.

At the family's Kalihi home on Wednesday, he rose from bed before noon, sleepy and rubbing at a pain in his leg. His mother, Lani, made him a bowl of eggs and rice, as Kevin picked up his game console.

"I used to play with my brothers, but I'm alone now," he said quietly, sitting on the couch.

Kevin and his oldest brother, Tuan, were toddlers when they were diagnosed with the disease. At the time, his mother was already pregnant with his youngest brother, Anthony. She made the difficult decision to have the baby, knowing there was a 50 percent chance that he would also be affected.

A flawed gene in patients with Hyper IgM Syndrome causes their bodies to produce an excess of some antibodies and not enough of others, leaving them prone to illness and infection.

The disorder affects only males, so Kevin's 10-year-old sister, Natalie, does not have symptoms but is a carrier of the genetic disease.

Four years ago, the three boys hoped to find a matching donor for a bone marrow transplant. The procedure could have saved their lives, but the window of opportunity in which they could have had the transplant passed without a match being found.

Tuan died in October 2006 after disease attacked his liver. Kevin and Anthony were both hospitalized last December, and Anthony succumbed that month.

Framed photos of Tuan and Anthony sit among candles, flowers and the boys' favorite soft drinks at an altar in the family's home.

"We were all scared, young," Kevin said.

"I miss my brothers a bit, yeah."

The family's story underscores the need for more members of ethnic minorities to join the national bone marrow registry.

The characteristics of bone marrow are genetically inherited, so a patient's best chance for a match is a donor within the same ethnic group, said Renee Adaniya, program manager for the Hawai'i Bone Marrow Donor Registry.

Adaniya estimated that minorities looking for a transplant may have a roughly 25 percent chance of finding a match, whereas that number for Caucasians may be 80 percent.

"There's a great need for minorities and people of mixed race to register," she said.

Because of his susceptibility to infection, it's difficult for Kevin to have the life of a normal 17-year-old. He likes to travel and has visited the Big Island, Louisiana and Connecticut — to see snow — but those memories are squeezed between memories of illness and trips to the hospital.

He is looking forward to going to McKinley to receive his certificate, but is more excited about getting his driver's permit and eventually his license.

"I might help my mom with doing some groceries, or my grandma," he said. "She always uses the bus to get to Chinatown."

Driving holds the possibility of going places, of seeing the mountains and the ocean, he said. Until then, he is trying to think of the perfect ending for his video game and its hero.

"In the end, he would find out most about himself, but it would still be a mystery," he said.

"Maybe there would be a sequel."